1. Field of the Invention
This invention relates to trimming of thermoformed articles from a plastic web. It particularly relates to precise trimming of such articles by obviating the effects of unequal spacings between the articles. It specifically relates to a method and apparatus for loosely indexing and aligning a plurality of thermoformed articles with article punches, isolating the articles from each other so that they exactly register themselves with the punches in a selected sequence, precisely trimming them from the plastic web, and ejecting them from the apparatus.
2. Brief Description of Prior Art Practices
Thermoformed plastic articles are currently trimmed from the surrounding web of sheet material by treating the web as a complete unit. There is consequently no freedom of movement of any article in relation to those surrounding it. As a result, accuracy and consistency of trim depend wholly on the indexing of the unit or web into the trim area and on the shrinkage values of the web that are built into the trim tool. However, shrinkage values for thermoformed plastic articles and for the surrounding web can vary with sheet characteristics, article size and shape, and conditions of thermoforming. In consequence, indexing of the articles and surrounding web into the trim area often becomes quite critical, so that trimmed edges of articles may be unsatisfactory and cause significant product rejects.
There is consequently a need for a method and apparatus that can obviate the present practice of estimating shrinkage values for indexing of the web into the trim area and for construction of the trim tool.
A potentially useful concept is disclosed in U.S. Pat. No. 1,062,028 for sheets of cooked pastry cones. When such sheets are removed from the cooking machine, more or less uneven shrinking of the pastry connecting the cones is found to have occurred, so that the cones are more or less unevenly interspaced. The disclosed trimmer comprises tapered plungers, disposed in rows of progressively decreasing length and having outwardly tapered spindles which successively center the cones in opposed tubes. The longer plungers trim the bottom row of cones without bringing any strain upon the remaining cones. Because the centers of the tubes are more widely interspaced than the centers of the baked cones, each succeeding row of cones is slightly raised when it is engaged by the plungers and forced into the tubes after the lower row has been sheared off.
This trimming operation is a discontinuous one, however, and seems to require a stiff and non-resilient sheet as pressure is transversely exerted by the tapered plungers. There is freedom of movement of the cones in the direction of decreasing length for the plungers. There is no freedom of movement side-to-side (across the web). This relieved-punch concept also requires that all cones along a row are in parallel.
U.S. Pat. No. 3,785,762 describes a thermoforming apparatus which includes a die-cutting press comprising a top and bottom platen which are driven by a toggle mechanism arranged so that the upper die comes down around the thermoformed article, providing a self-centering action, and then the lower platen rises to complete the cutting action. This self-centering action compensates for minor variations in the parts as they are molded and indexed, but it appears to be operable for a single thermoformed article per stroke and cannot compensate for minor variations in distances between a plurality of thermoformed articles being trimmed.
A hole-punching device comprising rigidly mounted cylindrical holes and adjustable ball punches, having a larger diameter than the diameter of cylindrical female die elements, is described in U.S. Pat. No. 4,123,956 for cutting a plurality of holes in a thin elastomeric material. However, the problems surmounted by this device are caused by the thinness and elasticity of the elastomeric sheet, and there are no preformed articles to be trimmed from the sheet.